Denial of service attack via push rule patterns in matrix-synapse

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.3 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2021-29471: 1.65% estimated exploitation activity in the next 30 days; percentile 76.49 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact "Push rules" can specify conditions under which they will match, including eventmatch, which matches event content against a pattern including wildcards. Certain patterns can cause very poor performance in the matching engine, leading to a denial-of-service when processing moderate length events. Patches The issue is patched by https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c. Workarounds A potential workaround might be to prevent users from making custom push rules, by blocking such requests at a reverse-proxy. For more information If you have any questions or comments about this advisory, email us at security@matrix.org.

Published by the source

13 May 2021, 20:22 UTC

Last modified by the source

8 Jul 2026, 06:27 UTC

Source last checked

26 Oct 2026, 13:21 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-x345-32rc-8h85

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-x345-32rc-8h85

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# Patch Next advisory brief: CVE-2021-29471

Advisory: Denial of service attack via push rule patterns in matrix-synapse
IDs: CVE-2021-29471, GHSA-x345-32rc-8h85
Record: https://patchnext.com/advisories/cve-2021-29471 (revision 3, source checked 26 Oct 2026, 13:21 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: matrix-synapse 1.33.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI matrix-synapse: affected < 1.33.2; fixed in 1.33.2

## Suggested inspection (read-only)
- `python -m pip show matrix-synapse`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on matrix-synapse.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-x345-32rc-8h85 (OSV record GHSA-x345-32rc-8h85 lists affected ranges: PyPI matrix-synapse >=0 <1.33.2 (fixed))
- Verified: https://github.com/advisories/GHSA-x345-32rc-8h85 (GitHub-reviewed advisory GHSA-x345-32rc-8h85 lists affected ranges: PyPI matrix-synapse >=0 <1.33.2)

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